In extracorporeal blood processing, blood is taken out of a human subject, processed (e.g. treated) and then reintroduced into the subject by means of an extracorporeal blood flow circuit (“EC circuit”) which is part of a system or machine for blood processing. Generally, the blood is circulated through the EC circuit by a blood pump. In certain types of extracorporeal blood processing, the EC circuit includes an access device for blood withdrawal (e.g. an arterial needle or catheter) and an access device for blood reintroduction (e.g. a venous needle or catheter), which are inserted into a dedicated blood vessel access (e.g. fistula, graft or catheter) on the subject. The access devices form a fluid connection between the EC circuit and the cardiovascular system of the subject. This type of EC circuit is, e.g., used in extracorporeal blood treatments such as hemodialysis, hemodiafiltration, hemofiltration, plasmapheresis, bloodbanking, blood fraction separation (e.g. cells) of donor blood, apheresis, extracorporeal blood oxygenation, assisted blood circulation, extracorporeal liver support/dialysis, ultrafiltration, heart congestion failure treatment, etc.
It is vital to minimize the risk for malfunctions in the fluid connection that may lead to a potentially life-threatening condition of the subject. A particularly serious condition may arise if the EC circuit is disrupted downstream of the blood pump while the blood pump is running, e.g. by the access device for blood reintroduction coming loose from the blood vessel access. Such a venous-side disruption, which is commonly referred to as a Venous Needle Dislodgement (VND), may cause the subject to be drained of blood within minutes. A disruption on the arterial side, e.g. by the access device for blood withdrawal coming loose from the blood vessel access, may also present a patient risk, by air being sucked into the EC circuit and transported into the cardiovascular system, causing air embolism.
Machines for extracorporeal blood treatment typically include a safety system that monitors the status of the fluid connection between the EC circuit and the subject and triggers an alarm and/or an appropriate safety action whenever a potentially dangerous situation is detected. In dialysis machines, it is common for such safety systems to operate on a pressure signal from a pressure sensor in the EC circuit, where the fluid pressure measured by the pressure sensor is responsive to a disconnection of the EC circuit from the blood vessel access. For example, the pressure sensor may be arranged to measure the pressure level on the venous side of the EC circuit. A venous-side disconnection results in a changed venous-side pressure, which may be detected by comparing the measured pressure level with one or more alarm thresholds that define a predefined, allowable pressure range.
Conventionally, the alarm thresholds are set, automatically by the machine or manually by an operator, and subsequently acknowledged by the operator at the beginning of a treatment session and may remain fixed throughout the session. The machine may allow the operator to manually change the alarm thresholds, and the machine may automatically change the alarm thresholds when the blood flow in the EC circuit is changed.
Generally, the alarm thresholds are primarily set to avoid false negatives in the VND detection, i.e. missed alarm conditions, and thus inevitably results in generation of false positives, i.e. false alarms. At the same time, it is important to avoid frequent false alarms that require the attention of dialysis personnel. A difficulty in this context is that the measured pressure level may change for other reasons than a VND during a treatment session, e.g. as a result of the patient moving, variations in the blood flow rate through the EC circuit, variations in the pressure drop in the access devices, variations in the composition of the blood (e.g. hematocrit), wear in the blood pump, changes in access pressure, etc.
It may be equally challenging to set alarm thresholds in other techniques that are used or proposed for monitoring of the status of the fluid connection between the EC circuit and the subject. Such other techniques may involve detecting pressure waves that have propagated via the fluid connection to a pressure sensor in the EC circuit, applying a blood leakage sensor onto the vascular access to detect presence of wetness or blood, operating an optical sensor attached to the patient to detect a perturbation transmitted from the apparatus to the patient via the fluid connection, analyzing an image signal from a camera directed to the vascular access, electrically detecting a disconnection of the access device from the vascular access, etc.
Although the foregoing description is given in the context of extracorporeal blood processing, it is understood that a corresponding need to detect a disruption of a fluid connection between two fluid containing systems may arise in other fields of technology.